1. Field of the Invention
The present disclosure relates to an image forming apparatus such as a copying machine and a laser printer.
2. Description of the Related Art
In a known electrophotographic color image forming apparatus, image forming units for forming yellow, magenta, cyan, and black images are independently provided for high speed printing, and the images are successively transferred from the image forming units to an intermediate transfer member, and are further transferred collectively from the intermediate member to a transfer material.
Japanese Patent Application Laid-Open No. 2004-29057 discusses an image forming apparatus using an endless intermediate transfer belt as the intermediate transfer member. A secondary transfer roller is used as a secondary transfer member, and one of support rollers supporting the inner peripheral surface of the intermediate transfer belt is used as a roller opposing the secondary transfer roller. The secondary transfer roller press-contacts the opposing roller across the intermediate transfer belt, forming a secondary transfer region where the secondary transfer roller and the intermediate transfer belt are held in contact with each other.
The transfer material is conveyed to the secondary transfer region where a difference in potential is generated between the secondary transfer roller to which a voltage is applied and the opposing roller which is grounded, and a toner image on the intermediate transfer belt is electrostatically transferred to the transfer material.
However, the image forming apparatus which forms the secondary transfer region with the intermediate transfer belt, the secondary transfer roller, and the opposing roller has the following problem.
The action of an electric field generated between the two rollers of the secondary transfer roller and the opposing roller is exerted not only on the secondary transfer region but also on the periphery thereof. The action of the electric field is also exerted at an inlet region situated on the upstream side of the secondary transfer region, so that, in some cases, before the transfer material enters the secondary transfer region, the toner image on the intermediate transfer belt is electrostatically moved onto the transfer material at the inlet region. As a result, the toner image is transferred to a position on the transfer material different from the predetermined position (the proper position to which the image ought to be transferred), so that the image is disturbed and splattered, resulting in deterioration in image quality. In the following, this phenomenon will be referred to as splattering.
To suppress the splattering phenomenon, according to Japanese Patent Application Laid-Open No. 2009-69466, in the vicinity of the secondary transfer region, a support roller is arranged on the upstream side of the opposing roller on the inner peripheral side of the intermediate transfer belt. To save space, this support roller is formed as a small diameter roller, and has a function to regulate the position of the intermediate transfer belt such that the recording material enters the secondary transfer region along the intermediate transfer belt. As a result, the splattering phenomenon is suppressed on the upstream side of the secondary transfer region.
However, in this construction, it is necessary to arrange a support roller on the upstream side of the secondary transfer region, resulting in a rather complicated construction.